MUG (4-MU-glcUA; 4-Methylumbelliferyl-β-D-glucuronic acid dihydrate), also known as MUGlcU, is a fluorogenic β-glucuronidase substrate that releases the blue fluorescent 4-methyl-7-hydroxycoumarinon enzymatic hydrolysis. The substrate is commonly used for identifying E. coli contamination and for detecting marker GUS gene expression in plants with high sensitivity.